1. Field of the Invention
The present invention relates to an Ag ball having a low alpha (α) dose; an Ag core ball having the Ag ball coated with a solder plating; a flux-coated Ag ball having the Ag ball coated with a flux layer; a flux-coated Ag core ball having the Ag core ball coated with the flux layer; a solder joint using the Ag ball, the flux-coated Ag ball, the Ag core ball and a flux-coated Ag core ball; a formed solder using the Ag ball, the flux-coated Ag ball, the Ag core ball and the flux-coated Ag core ball; a solder paste using the Ag ball, the flux-coated Ag ball, the Ag core ball and the flux-coated Ag core ball; and an Ag paste using the Ag ball.
2. Description of Related Art
Recently, along development of compact information equipment, electronic components to be mounted on them have been downsized rapidly. A ball grid alley (hereinafter referred to as “BGA”) having electrodes at its rear surface is applied to such electronic components in order to satisfy a requirement of a narrowed connection terminal and a reduced mounting area because of the downsizing requirement.
The electronic components using the BGA are, for example, semiconductor packages. In the semiconductor packages, semiconductor chips having electrodes are sealed with resin. Solder bumps are formed on the electrodes of the semiconductor chips. The solder bump is formed by joining a solder ball to the electrode of the semiconductor chip. The semiconductor package having the BGA is mounted on a printed circuit board such that each solder bump touches a conductive land of the printed circuit board. Then, the solder bump and the land are joined by melting the solder bump with heating so that the semiconductor package is mounted on the printed circuit board. Recently, a three-dimensional high-density package is studied by stacking up the semiconductor packages in a height direction in order to meet the further high-density mounting.
However, if the BGA is applied to the semiconductor package of the three-dimensional high-density mounting, the solder ball may be crushed by a semiconductor package's weight and a shorted circuit occurs between the electrodes. This affects the high-density mounting.
Therefore, another type of the solder bump is investigated wherein the bump is a ball having a small diameter and is formed by a metal such as Ag, etc. having a melting point more than the solder. Even if the weight of the semiconductor package is loaded to the solder bump having the Ag ball when the electronic component is mounted on the printed circuit board, then the Ag ball can support the semiconductor package because the Ag ball does not melt at the melting temperature of the solder. Therefore, the solder ball is not crushed by the semiconductor package's weight. A related technology is disclosed in, for example, Japanese Patent Application Publication No. 2011-214040 (hereinafter referred to as “JP2011-214040 publication”).
By the way, the downsizing of the electronic components allows the high-density mounting, but the high-density mounting causes soft error problems. The soft error causes a possibility of rewriting a storage content of a memory cell in a semiconductor integrated circuit (IC circuit) with an alpha ray entering the memory cell. It is conceivable that the alpha ray is emitted by alpha decay of radioactive elements such as U, Th, Po in the solder alloy. Thus, solder materials having low alpha dose are being developed recently by reducing contents of the radioactive elements.
Therefore, Ag and Ag alloy having the low alpha dose are required as discussed in the JP2011-214040 publication.
However, it has not been considered to balance the alpha dose and a sphericity of the Ag ball until now wherein the sphericity represents a deviation from a true sphere. Therefore, the soft error problem was not resolved. A cause of this problem was that a standoff height was controlled when the solder bump was formed, there were the radioactive elements and isotopes in the impurities in the Ag ball, and these elements and isotopes were diffused to emit the alpha ray after the soldering joint of the Ag ball. Then, the alpha ray emitted from the Ag ball entered the memory cell of the semiconductor chip and the soft error occurred.
The Ag ball having the high sphericity and the low alpha dose is required as discussed hereinbefore; however, no prior art including the JP2011-214040 publication did not discuss at all the Ag ball having both the high sphericity and the low alpha dose. The Ag ball was manufactured traditionally by heating the Ag material to a temperature equal to or more than 1,300° C. and melting the material. Thus, it was conceivable that a content of the radioactive isotope such as Po of emitting the alpha ray was reduced significantly by the volatilization so that the alpha ray of Ag might not affect the soft error.
However, it was not proved in the prior art that the manufacturing condition for the Ag ball caused to reduce the alpha ray from the Ag ball to prevent the soft error. Since a boiling point of 210Po is 962° C., it may be volatized sufficiently not to generate the soft error if heating to a temperature equal to or more than 1300° C. However, since the heating at the manufacturing process of the Ag ball did not purpose to volatilize Po, there was no guarantee to reduce the amount of 210Po at this temperature. Thus, it is doubtful that the conventional manufacturing process of the Ag ball produces the Ag ball having the low alpha dose.
It is conceivable to manufacture the Ag ball by using the Ag material having a high purity. However, in this case, it is not required to reduce a content of elements not to participate the alpha dose of the Ag ball. When the Ag material having the high purity is used without thought, its cost only rises.
If the sphericity of the Ag ball is low, the original function of the Ag ball does not work to control the standoff height when forming the solder bump. In this case, the bumps having uneven heights are formed so that a problem occurs at the mounting process.